inati
First Post
An NPC that I was cooking up was a Gnome Ranger that tinkers a lot (classic gnome) and uses a double barrel gun so that he can perform double shot attack powers. He would be one of those Gadgeteer Geniuses, so he would be one of the few in the world to have a gun, but your setting sounds more like guns are more common than that! 
For "magic" properties of the weapon, such as a +1 weapon, I would have him tinker with the gun and create a scope for better accuracy, and a new formula of gunpowder for more damage. Other magically properties could also be explained in similar ways, such as adding magnesium to rounds for "flaming" weapons.
You might also want to look at the rules regarding the Repeating Crossbow, found in the Adventurer's Vault (pg 10):
Prof +2 Dam 1d8 Range 10/20 Weight 6lbs Group Crossbow Prop Load Free
We can see it gives us rules for Magazines, specifically their cost, ammo capacity, and what is required to load a new magazine into the weapon. I feel that modifying and incorporating such rules into a setting with guns should be considered, as it makes for a lot more elegant solution than trying to explain muzzle loading guns and the time it takes to reload them....unless that's the flavor you want for your campaign!

For "magic" properties of the weapon, such as a +1 weapon, I would have him tinker with the gun and create a scope for better accuracy, and a new formula of gunpowder for more damage. Other magically properties could also be explained in similar ways, such as adding magnesium to rounds for "flaming" weapons.
You might also want to look at the rules regarding the Repeating Crossbow, found in the Adventurer's Vault (pg 10):
Stats:Repeating Crossbow: A rectangular magazine
attaches to the top of this crossbow. A double-action lever drops a bolt into place as a free action, then fires it as a standard action. A repeating crossbow does not need to be reloaded as long as it has ammunition
in its magazine. A magazine costs 1 gp and holds 10 bolts. It takes a standard action to remove an empty magazine and load a new one.
Prof +2 Dam 1d8 Range 10/20 Weight 6lbs Group Crossbow Prop Load Free
We can see it gives us rules for Magazines, specifically their cost, ammo capacity, and what is required to load a new magazine into the weapon. I feel that modifying and incorporating such rules into a setting with guns should be considered, as it makes for a lot more elegant solution than trying to explain muzzle loading guns and the time it takes to reload them....unless that's the flavor you want for your campaign!